Western researchers have called for tighter curbs on Indian clinics making "extravagant claims" over the use of embryonic stem cells after a Delhi doctor this week said she had treated 100 terminally ill patients with the therapy.

Experts in Britain expressed concern and scepticism on hearing that Geeta Shroff, 41, who runs a genetics research laboratory and hospital in south Delhi, had produced and purified stem cells from human embryos. They said it was "highly improbable" that Dr Shroff had, as she claimed, treated in this way a variety of incurable conditions ranging from renal failure to cerebral palsy.

The controversial claims appear to be the boldest yet by mavericks working with stem cells in countries stretching from Mexico to China, who cast themselves as crusaders against a medical establishment that has little to offer but comfort to terminally ill patients.

"My patients often have no other choice. I am their last chance," said Dr Shroff in an interview with the Guardian.

Scientists say the technology is so new that anyone can claim potential for it. But without clinical trials it is difficult to know if it works. Under Indian law doctors may treat "incurable and terminally ill" patients with novel procedures.

Dr Shroff points to satisfied patients but there is no scientific proof that her therapy works. She would not discuss how her stems cells were purified or tested. There was no explanation of how the cells functioned in the body. Her clinic has not run any tests to analyse how the body is affected by the treatment, and western experts say that, potentially, she could be pushing an important therapy into the "realms of quackery", that her work is tantamount to "human experimentation".

Stephen Minger, director of the King's College London stem cell biology laboratory, said: "It is highly implausible and frankly downright dangerous. If the Indian government wants to promote stem cell research then it needs to seriously look at regulation of these doctors and if necessary close them down."

An editorial in the British Medical Journal this year warned that Indian authorities needed to act to "prevent the escalating numbers of clinics offering stem cell cures for all sorts of ills".

Professor Minger said: "Only one team in the world, from the American firm Geron, is pushing for clinical trials and that is a very well understood, specific application for spinal cord injury. And we have concerns with that. This doctor has not published a peer-reviewed paper. How can one independently verify the results?"

However, Dr Shroff said patients from the US and Britain were going to her for treatment. Her therapy appears to consist of injecting a clear liquid, containing, she says, millions of cells cultivated from a human egg, into a bicep or base of the spine. Dr Shroff said her embryos were obtained with consent.

The Guardian spoke to a number of Dr Shroff's patients before this week's announcement. They claimed their conditions had improved because of her treatment. Poonam Singh, 25, contracted TB of the spine 11 years ago and was paralysed from the waist down. After a month of injections, she believes her condition is improving. "I can sit up, feel sensation in my legs. I could not lift my legs, now I can take a few steps," she said.

Jaspal Toor, a Briton with motor neuron disease, a terminal condition, had trouble breathing and slurred speech. Ms Toor, who paid £2,000 for a year's treatment, says that after a series of injections in the summer she could breathe more easily, and now swallows and talks without difficulty.

The clinic could not provide any detailed biochemical explanation for what had happened in these cases. Scientists in the west suggested that what was being seen was a placebo effect.

Embryonic stem cells can make any type of cell in the body and can develop into any of the body's organs. The accepted wisdom is that it is very difficult to control what type of specialised cells develop and, if you generate one cell, often it is part of a messy jumble containing unwanted cells. "Putting in undifferentiated stem cells carries a serious risk of tumours and cancer," said Simon Best, of the UK Biotechnology Association. "This kind of work needs proper oversight because people's lives are at stake."

Researchers in Britain say stem cell therapies are a decade away. They add that in the west and advanced economies of Asia there are legal constraints that keep scientists from rushing ahead with treatments before they have been "thoroughly tested for safety and effectiveness".

Alison Murdoch, of Newcastle University Fertility Centre, which made headlines in May by cloning embryos, said: "Desperate patients might be tempted but false hope is not hope. There are all sorts of ethical issues that this kind of unregulated work throws up. There are no animal tests, no checks on where the embryos are coming from."

Dr Shroff defends her actions, saying everything she does is within India's guidelines and that she informed the authorities. "Everything I have done has been notified to the Indian Council of Medical Research and there has been no comeback," she said. The ICMR said it had been informed but had not given consent. Prasanna Hota, the top civil servant in India's health ministry, said: "We have our concerns and worries about Dr Shroff's work."

Profile

· Geeta Shroff operates from a three-storey, 20-bed hospital, Nu Tech Mediworld, in south Delhi. Hanging from her office walls are Indian medical diplomas, training certificates from Asian research institutes, and a picture of her with India's prime minister Manmohan Singh, who is a friend of the family

· Dr Shroff graduated from Delhi University's school of medical science in 1993 before joining Vera Hingorani, a pioneering physician, at India's top medical institute, the All India Institute of Medical Sciences

· She shot to fame in the early 90s by developing a technique for determining a baby's sex while in the womb without taking a scan.A government clampdown, however, put an end to that revolutionary practice and Dr Shroff settled into treating infertile couples until she began working on stem cells in 1999. Six years later she finds herself back in the headlines.